Abstract
During three low-altitude (99, 66, 66 km) flybys through the Enceladus plume in 2010 and 2011, Cassini's ion neutral mass spectrometer (INMS) made its first high spatial resolution measurements of the plume's gas density and distribution, detecting in situ the individual gas jets within the broad plume. Since those flybys, more detailed Imaging Science Subsystem (ISS) imaging observations of the plume's icy component have been reported, which constrain the locations and orientations of the numerous gas/grain jets. In the present study, we used these ISS imaging results, together with ultraviolet imaging spectrograph stellar and solar occultation measurements and modeling of the three-dimensional structure of the vapor cloud, to constrain the magnitudes, velocities, and time variability of the plume gas sources from the INMS data. Our results confirm a mixture of both low and high Mach gas emission from Enceladus' surface tiger stripes, with gas accelerated as fast as Mach 10 before escaping the surface. The vapor source fluxes and jet intensities/densities vary dramatically and stochastically, up to a factor 10, both spatially along the tiger stripes and over time between flyby observations. This complex spatial variability and dynamics may result from time-variable tidal stress fields interacting with subsurface fissure geometry and tortuosity beyond detectability, including changing gas pathways to the surface, and fluid flow and boiling in response evolving lithostatic stress conditions. The total plume gas source has 30% uncertainty depending on the contributions assumed for adiabatic and nonadiabatic gas expansion/acceleration to the high Mach emission. The overall vapor plume source rate exhibits stochastic time variability up to a factor ∼5 between observations, reflecting that found in the individual gas sources/jets. Key Words: Cassini at Saturn—Geysers—Enceladus—Gas dynamics—Icy satellites. Astrobiology 17, 926–940.
Highlights
The Cassini spacecraft’s 2005 discovery of geyser emissions at Enceladus’ south polar region (Dougherty et al, 2006; Hansen et al, 2006; Spahn et al, 2006; Tokar et al, 2006; Waite et al, 2006), from surface hot spots along parallel, elongated ‘‘tiger stripe’’ surface troughs (Porco et al, 2006; Spencer et al, 2006) was a major milestone in understanding both the physics of Saturn system and the likelihood and frequency of present-day geological activity at solar system icy satellites
The uncertainty is acute in the high Mach number component of the emission, since it is not clear whether the gas is accelerated purely by passive adiabatic expansion of vapor from a static source, or whether the fluid acceleration is augmented by tidally driven compressional stress on the fissures (Kite and Rubin, 2016)
If tidally driven flow and boiling contribute to the acceleration, the speed may be much higher; for example, boiling liquid rising close to the surface may yield vapor escaping to space near 273 K, which (for Mach 16 · (18/44)1/2 & 10 flow as seen by ultraviolet imaging spectrograph (UVIS)/ion neutral mass spectrometer (INMS)) implies a flow speed of *6 km/s
Summary
The Cassini spacecraft’s 2005 discovery of geyser emissions at Enceladus’ south polar region (Dougherty et al, 2006; Hansen et al, 2006; Spahn et al, 2006; Tokar et al, 2006; Waite et al, 2006), from surface hot spots along parallel, elongated ‘‘tiger stripe’’ surface troughs (Porco et al, 2006; Spencer et al, 2006) was a major milestone in understanding both the physics of Saturn system and the likelihood and frequency of present-day geological activity at solar system icy satellites. Both flybys took place along similar north to south trajectories and thereby encountered the plume after closest approach, sampling the plume density and composition as the spacecraft traveled outbound from the south polar region In their analysis of these two flybys, Teolis et al (2010) modeled water vapor adsorption on the walls of the INMS gas inlet thermalization antechamber and found that such sticking introduced a time delay and distortion in the INMS H2O data. Beginning with the 91 km E7 flyby on November 2, 2009, the Cassini spacecraft carried out a series of low-altitude (
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